The ready availability of glucose and the superior sweetening power of syrups containing fructose have long made it desirable to find a practical method for converting glucose to fructose. The alkaline isomerization of glucose to fructose has been extensively investigated by many workers but the yields obtained from such a reaction are less than satisfactory due to the formation of significant amounts of unwanted by-products. In recent years, however, an alternative method has received considerable attention which is based on the discovery that certain microorganisms are capable of elaborating an enzyme which isomerizes glucose to fructose. U.S. Pat. Nos. relating to enzymatic glucose isomerization processes include 2,950,228, 3,616,221, 3,622,463, 3,623,953, 3,625,828, 3,645,848, 3,654,080 and 3,689,362.
In spite of the promising aspects offered by the enzymatic approach to glucose isomerization, production costs and efficient utilization of glucose isomerase enzyme preparations present formidable obstacles to the development of a commercially practical glucose isomerization process. Since most of the microorganisms known to produce glucose isomerase involve intracellular isomerase, the present state of technology relating to the production and use of cell-free isomerase preparations makes the adoption of a glucose isomerization process based thereon economically unattractive. Consequently, primary interest has centered on processes which utilize the isomerase-containing microbial cells in such a way that repeated use of the enzyme is achieved.
U.S. Pat. No. 3,694,314 describes one such process in which a Streptomyces organism is cultured under aerobic conditions and the cellular material is treated with filter aid before harvesting same by vacuum filtration using a rotary drum vacuum filter. The filter cake is subsequently dried, then slurried with a glucose-containing solution that is pumped through a pressure leaf filter to give a relatively thin layer of cell-containing material on the leaves of the filter. A glucose solution is then passed through the pressure leaf filter to obtain an effluent syrup containing mixtures of glucose and fructose. Although this process is reasonably effective, the degree of control is less than satisfactory in that cellular material bed depths and glucose solution flow rates must be maintained within certain limits in order to achieve acceptable operation. Equipment and maintenance costs relating to the process apparatus represent further disadvantages.
This invention provides a simple but effective means for immobilizing glucose isomerase so that it may be used repeatedly in either a continuous- or batch-type process. Moreover, the properties of the immobilized isomerase are such that relatively simple apparatus can be used to carry out the glucose isomerization. The physical properties and enzymatic activity of the immobilized isomerase material produce excellent glucose conversion rates at relatively high flow rates of substrate solution.